Add viewContainer contribution point

[thegecko]

As discussed in #200880 we are keen to add a contribution point to the debug viewlet.

This proposal implements this as well as allowing contributions to other view containers.

To use this proposal, the package.json would look similar to:

{
        ...
	"enabledApiProposals": [
		"viewContainerContext"
	],
	"contributes": {
		"commands": [
			{
				"command": "proposed.dummyMenu",
				"title": "Dummy Menu",
				"icon": "$(beaker)"
			}
		],
		"menus": {
			"view/container/title": [
				{
					"command": "proposed.dummyMenu",
					"group": "navigation",
					"when": "viewContainer == workbench.view.debug"
				}
			]
		}
	},
        ...

An example can be found here: https://github.com/thegecko/vscode-proposed-debug

Partially fixes: #200880